Crosstalk between cells of the bone microenvironment and hematopoietic cells affects each other's behavior, but a significant knowledge gap exists in the basic biology of the interactions and communication between osteolineage cells and hematopoietic stem cells (HSCs) in the adult. Filling this knowledge gap is important, as it has potential significance for understanding how healthy HSCs are maintained or altered in diseases of the bone and with ageing. The overall goal of this project is to identify the mechanisms that control HSC maintenance and differentiation in the bone marrow, and define which of these mechanisms are affected by cell-extrinsic changes in the bone. Sclerostin (abbreviated Sost for the gene, SOST for the protein) is a novel mediator of cellular communication between the skeletal and hematopoietic systems. SOST is a secreted protein that is produced primarily by osteocytes (OCYs), the mineralized bone cells embedded within the bone. SOST is a Wnt signaling antagonist that is critical for normal bone homeostasis. In Sost-knockout (SostKO) mice, the transition from osteoblasts to OCYs is uncontrolled, leading to massive increases in bone mass. Previous studies connected the misexpression of other secreted Wnt antagonists with dysregulated bone homeostasis and poor maintenance of long-term HSCs (LT-HSCs). However, the role of SOST on LT-HSC maintenance, cell cycling and differentiation still remains an open question. The adult bone marrow contains non-hematopoietic niche cells that support HSCs, such as mesenchymal stem cells (MSCs), osteoblasts (OBs) and endothelial cells (ECs). MSCs and OBs play dual roles as progenitors of OCYs and as niche cells for HSCs. SostKO mice display increased numbers of MSCs, OBs and ECs, but the effects the loss of Sost on their functional ability of maintain LT-HSCs are not known. The proposed studies will test the hypothesis that dysregulated bone homeostasis caused by Sost-depletion alters bone microenvironments in ways that directly affect the maintenance of LT-HSCs. The SostKO mouse model will be utilized as recipients in serial LT-HSC transplantation assays to discriminate between permanent or temporary changes of LT-HSC behavior after exposure to SostKO microenvironments. Cell-intrinsic characteristics of LT-HSCs that change due to the loss of Sost in the BM niche, such as altered LT-HSC cell division rates, levels of Wnt signaling activation, and evidence of epigenetic changes to the LT-HSC genome will be evaluated. In parallel, flow cytometric sorting will be used to dissect the distinct roles of SostKO MSCs, OBs, and ECs in the regulation of LT-HSC fate. Short-term in vitro co-culture assays will distinguish whether it is the interactions of LT-HSCs with MSCs, ECs, and/or OBs that influences LT-HSC fate, and gene expression analysis in sorted niche cell populations will identify differentially expressed genes that are involved in HSC maintenance. Taken together, the proposed studies will generate important new information on how Sost-deficiency affects LT-HSC maintenance and define the molecular and cellular players that are involved in HSC/niche crosstalk.